NEUTRAL HYDROGEN IN THE DIRECTION OF THE VELA SUPERNOVA REMNANT

We have carried out a study of the distribution and kinematics of the neutral hydrogen in the direction of the Vela supernova remnant (SNR). A field of 6.degrees 8 x 5.degrees 4 centered at l = 264.degrees 1, b = - 1.degrees 6 was surveyed using the Parkes 64 m radio telescope (h alf-power beamwidth 14.'7 at 21 cm). Nearly 2300 H I profiles were obt ained with a grid spacing of 7.'5. The presence of a thin, almost circ ular H I shell, centered at v = 1.6 +/- 0.8 km s(-1), is revealed. Thi s shell delineates the outer border of the X-ray emission as shown in the ROSAT observations of Aschenbach, Egger, & Trumper and wraps aroun d the receding part of the remnant. In addition, two higher velocity f eatures possibly associated with Vela are observed at about -30 and 30 km s(-1). These features are interpreted as gas accelerated by the ex pansion of the supernova shock. The low systemic velocity observed sug gests a distance shorter than 500 pc for the Vela SNR. The H I shell i s similar to 7 degrees in diameter and expands at v similar to 30 km s (-1). By assuming a distance of 350 pc, we calculate for this shell a linear radius of 22 pc, a swept-up mass of similar to 1200-2300 M-., a nd an atomic preshock density of similar to 1-2 cm(-3). The kinetic en ergy transferred by the supernova shock into the interstellar medium i s similar to(1-2)x 10(49) ergs, while the initial energy of the explos ion is estimated to be similar to(1-2.5) x 10(51) ergs. We present the distribution of the column density of the neutral material absorbing the X-radiation, an essential parameter in the analysis of X-ray data. A comparison between the H I and H alpha emission suggests that the H I shell contains embedded dust that might be responsible for increase d optical absorption in this region. On the other hand, the brightest are-shaped optical filaments associated with the western side of Vela show good correspondence with the H I features. From a comparison betw een the H I and Molonglo Observatory Synthesis Telescope 843 MHz radio continuum emission, we find that the outermost arched radio filaments correlate well with the main ridge of the H I shell. No strong inhomo geneities were found in the ambient H I medium in the direction of Vel a X (the central nebula, powered by the pulsar PSR B0833-45).